1. Field of the Invention
The invention pertains to the semiconductor field. More particularly, the present invention relates to output buffers for semiconductor devices, such as semiconductor memories.
2. Description of the Related Art
Two of the key specifications for synchronous static random access memories (SRAM) are the time from a clock pulse to a high impedance, or high-Z state (commonly referred to as the Tchz parameter), and the time from a clock pulse to a driven, or low-Z state (commonly referred to as the Tclz parameter). The ability to place outputs in a high-Z state is necessary to connect several devices (e.g. memory devices), or banks of such devices, to a common bus. For example, the outputs of a memory device should be tri-stated for a write or de-select cycle, but not for a read cycle. Moreover, in order to avoid bus contention issues during bank selection, it is important that the time from clock to high-Z be as short as possible. A short Tchz parameter allows the Tclz parameter to be shortened without causing bus contention. This results in higher performance for the system. Devices with short Tchz parameters may be electrically connected and disconnected from the bus rapidly and efficiently.
Traditionally, however, to determine whether to switch a bank or a device into a high impedance state or from a high impedance state to a driven state requires the combination of many control inputs. Indeed, whether to place the outputs of a device in a high-Z state or a driven state conventionally depends on whether the device or bank is currently selected or currently deselected, the state of a number of write control and read control signals, and the state of a plurality of other control signals. Thus, a complex combinatorial or other circuit is typically required to determine whether and when the outputs of a particular device are to be placed in a high-Z state, or are to be driven by the device.
For example, the mode of operation (e.g. read, write or de-select) of a memory device may be a function of a plurality of chip enable input signals, a plurality of write enable input signals, and a plurality of address strobe input signals. A significant component of the Tchz and Tclz times are a result of the combinatorial logic used to determine whether or not the outputs should be tri-stated for the current cycle. Thus, Tchz and Tclz are typically related to the sum of the propagation delays of the respective logic gates used to determine whether the outputs should be tri-stated.
This delay in determining whether outputs of memory devices should be tri-stated is a significant factor in the performance of such devices. Indeed, the ability to rapidly switch between a plurality of devices attached to a common bus directly affects the speed at which data may be accessed and transferred over the common bus. For example, to switch from a first device to a second device connected to a common bus, the outputs of the first device must first be placed in a high impedance state before the second device may be selected and taken out of tri-state. Improper timing may cause the outputs of both devices to be active at the same time, resulting in bus contention issues and abnormally large current flowing through the bus structure. Alternatively, a slow clock to low impedance parameter Tclz will delay the second device from switching out of the high impedance state, thus reducing the overall performance of the memory device.
There has been a long felt need, therefore, for means and a method that would allow rapid switching between outputs of devices connected to a common bus structure. This would be possible if the outputs of, e.g. memory devices, could be quickly placed in and taken out of a high impedance state. What is also needed, therefore, are a means and a method for quickly tri-stating outputs of semiconductor devices. Furthermore, means and methods are needed that reduce the time necessary to switch into and out of tri-state. Such means and methods should not, however, adversely affect the functionality and performance (e.g. access time) of the device(s) in question.